The present invention relates to a brake fluid composition consisting essentially of a copolymer of alkylmethylsiloxane and dimethylsiloxane. It further relates to a method of employing said copolymer composition in an automotive brake system.
The benefits of silicones for use as hydraulic and brake fluids were recognized early in the development of these unique materials. McGregor et al., U.S. Pat. No. 2,398,187, disclosed the broad generic use of dialkyl siloxane polymer and copolymer fluids for such applications on the basis of their slight change in viscosity with temperature, low pour point and high flash point. In addition, McGregor et al. recognized other beneficial properties, including: low volatility, low hygroscopicity, little or no corrosive or decomposing effect upon metal and rubber hydraulic device components, and little or no gasification or solidification tendencies under the higher or lower temperature conditions encountered in the various types of hydraulic applications.
More recently, commercial attention has focused on polydimethylsiloxanes (PDMS) as viable alternatives to the glycol-based polyethers traditionally employed in automotive brake fluid systems. This is not surprising, since PDMS represents the most ubiquitous and inexpensive silicone of manufacture. Its virtues as a brake fluid, relative to the glycols, have been extolled, e.g., by G. R. Browning (Paper 740128 presented at SAE Automotive Engineering Congress, Detroit, February, 1974). This reference points out a further key advantage of a high boiling polydimethylsiloxane, namely that this fluid will not absorb or dissolve a significant amount of water, contrary to a glycol fluid. Such inevitable humidification of glycol fluids with time has been found to result in reduced boiling point and, ultimately, to lead to the potentially dangerous condition of vapor lock and loss of brake capability.
These, and other considerations of beneficial properties have helped pave the path for enactment of a separate category of specifications by the Department of Transportation for low water tolerant brake fluids, known as DOT 5. These requirements are further defined by Federal Motor Vehicle Safety Standard No. 116 and were published in the Federal Motor Vehicle Safety Standards and Regulations, Supplement 80, Oct. 23, 1974, which is hereby incorporated by reference and henceforth referred to simply as DOT 5.
Notwithstanding all their virtues, polydimethylsiloxane fluids retained one major disadvantage for certain specialized applications. Their relatively high (about -50.degree. C.) solidification temperature precluded their acceptance for utilization as an all-around (arctic/tropic) military brake fluid. Such disadvantage was engineered out of these fluids by copolymerizing mono- and trifunctional units with the difunctional siloxanes to attain a non-regular structure (i.e., a non-linear siloxane) and thus greatly reducing the tendency to crystallize. Silicone brake fluids of this capability have also been defined in terms of rigid specifications, this time by Military Specification MIL-B-46176, which is hereby incorporated by reference and henceforth simply referred to as MIL. This specification is more demanding than DOT 5, particularly in the areas of low temperature fluidity/appearance, flashpoint, vapor lock temperature and swell of rubber seal components.
An example of a non-linear siloxane was disclosed by Holbrook et al. in U.S. Pat. No. 4,137,189. In this case, the object was to provide an hydraulic fluid which could be utilized in all fluid transmission systems of an automobile. This application demanded greater lubricity on metals than ordinarily required in brake fluid applications. It should, however, be recognized that some lubricity is needed to prevent scoring of cylinders in brake systems. Generally, the lubricity of polydimethylsiloxanes is only marginal. Thus, Holbrook et al. disclosed a composition consisting essentially of a non-linear siloxane fluid, a chlorendate diester and a lubricant additive selected from dithiocarbamates and phosphorodithioates of antimony and lead.
A variation on the teachings of Holbrook et al. was disclosed by Keil in U.S. Pat. No. 4,443,351, wherein it was suggested that the less expensive, linear polydimethylsiloxanes, or copolymers of PDMS with alkylmethyl siloxanes, be blended with a chlorendate diester, a lubricant additive (as above) and a block copolymer of polydimethylsiloxanepolybutadiene. Such compositions were utilized as concentrates of said lubricant additives, which could be added to other siloxane hydraulic fluids or used independently, and provided settling stability (of the additives) over a wide temperature range.
Yet another means of enhancing the lubricity of siloxanes was disclosed by Brenner in U.S. Pat. No. 3,671,433. Alkylmethyl polysiloxane fluids were mixed with small quantities of dodecenyl succinic acid. It was shown that when the alkyl groups contained from 6 to 18 carbon atoms superior lubrication properties resulted. Unfortunately, in addition to being considerably more expensive to produce than PDMS, these fluids have been shown to cause unacceptably high swell in rubber seal components. Moreover, they generally do not pass DOT 5, much less MIL specifications, making them less attractive brake fluid candidates.
It is thus recognized that high boiling polydimethylsiloxane fluids demonstrate great advantage in applications requiring a stable and versatile automotive brake fluid. Yet, as automobile designs tend towards more efficient and compact engines and brake systems, there is increasing need for improved brake fluids which can operate at the higher temperatures associated with such congested surroundings. This is further underscored by the significant temperature elevation imparted to brake fluids during, e.g., prolonged start-stop driving or descent on long, steep grades. Holbrook (Paper 810803, presented at SAE Passenger Car Meeting, Dearborn, Mich., June, 1981), for example, has shown that such driving conditions can lead to fluid temperature increases of well over 100.degree. C. above ambient.
Thus, once mechanical aspects of an automotive brake system have been determined, there remain three key temperature-related contributions to the total brake pedal travel (brake system compliance) due to the brake fluid itself: (1) compressibility of the fluid, (2) volatility of the fluid and (3) dissolved permanent gases, typically air, in the fluid. Although it is generally desirable to keep the brake system compliance as low as possible for a highly responsive brake system, variation in compressibility is generally smooth over the temperatures in question, and is, therefore, of lesser concern than the other two factors. The second factor has already been mentioned in the form of vapor lock, wherein some component (or indeed the fluid itself) suddenly vaporizes (boils) to form a vapor pocket due to high temperature. The last category, as far as is known to applicants, has not been addressed prior to the instant invention, but can potentially lead to increased pedal travel. For example, the fluid in an hydraulic line in proximity to a disk brake may be rapidly heated by repetitive braking. Consequently, some of the dissolved air must come out of solution to establish a new (lower) equilibrium air concentration in the fluid. If such a change is sufficiently abrupt, bubbles can nucleate to form air pockets in the bulk fluid. Such air pockets would have to be compressed out before a caliper (fitted with brake pads) could be activated by the brake pedal to frictionally engage the brake disk. This, in turn, could result in loss of braking performance. Furthermore, reduction of ambient atmospheric pressure could induce a similar undesirable effect. Thus, despite all the advantages of PDMS in brake fluid applications, these materials, whether branched or linear, allow considerably greater air solubility than the conventional glycol-based fluids.
It has now been discovered that significant reduction in gas, typically air, solubility can be achieved in certain random copolymers of dimethylsiloxane with alkylmethylsiloxane. Moreover, this can be accomplished while retaining other desirable features of PDMS fluids for brake fluid applications. In other words, these fluids can still be formulated to meet the requirements set forth in the aforementioned DOT 5 and MIL specifications, and, indeed, often surpass them. The potential benefit of such a reduction in air solubility is an improved margin of safety under harsh driving conditions, such as recited above.